Not necessarily. With any product that has expectations attached to it, there will always be a desire for a high box office gross. So if it doesn't meet the figure they're wanting, they may just go back to a regular MOS sequel, or go back to Batman solos.

This film, like MOS, is a tent-pole film intended to be a backbone of Warner's future line-up. I imagine that if it doesn't make at least $800 million, that plans for a Justice League film could change.

I know this isn't a completely apt comparison, but look at Jack Reacher. Even with it being a Tom Cruise, receiving generally good reviews and only costing $60 million, Paramount was very reticent about greenlighting a sequel, and didn't do it until it made over $200 mil, which is over 3x its cost.

A studio always has expectations, and if they aren't met, even if it's brand name, a sequel is by no means guaranteed.

Mark my word, that will be the case. If "Superman/Batman" costs the same as Man of Steel did (around $225 million) then they would only need to make more than $411 million to get a sequel (not $1 billion) even though fans will be comparing it to Marvel's "Avengers" or "The Dark Knight" as far as box-office. With that being said, I do not expect that this will will be produced for anything less than $350 million, so the bar will be higher (more like $650 million or more).